Why can I not shift my Hyundai accent automatic transmission into drive?

Sounds like my son encoutered the same problem tonight with his Hyundai 2001 Accent GL - 1.6 llitre. With aplying the brake, pushing the unlock shifter button, the shifter won't move. This happened when he started his car tonight (real cold night -23 Celc.). Is this electical or mechanical related; perhaps a fuse is blown although which one? Any help would be greately appreciated. Thanks your shift lock may be seizing up it is right next to the shifter underneath the console